1. Field of the Invention
The present invention relates to an organic electroluminescent element (hereinafter called "organic EL element").
2. Description of the Related Art
There is an organic EL element with a double-layer structure, which has an organic fluorescent film 3 as a light-emitting layer and an organic hole-carrying layer 4, both formed of an organic compound and laminated one on the other between a metal electrode 1 as a cathode and a transparent electrode 2 as an anode. The organic hole-carrying layer 4 has a function to facilitate the injection of holes from the anode and a function to block electrons. A glass substrate 6 is arranged outside the transparent electrode 2. As electrons from the metal electrode 1 and the holes from the transparent electrode 2 are recombined, excitons are generated. The excitons emit light while they are deactivated, and this light is emitted outside through the transparent electrode 2 and the glass substrate 6.
It is desirable that the metal electrode 1 as the cathode in the conventional organic EL element be made of metal having a low work function equal to or below 3 electron volts (which will hereinafter be referred to as "low work function metal") due to its effectiveness in electron injection. Since the cathode of such low work function metal is disadvantageous in film formability, stability and the like, however, metals having a high work function above 3 electron volts (which will hereinafter be referred to as "high work function metal"), for example, aluminum or magnesium or magnesium indium alloy, magnesium aluminum alloy or magnesium silver alloy as a single material or a co-evaporated alloy are presently used (see U.S. patent application Ser. No. 135,128 filed Feb. 11, 1987, corresponding to Unexamined Japanese Patent Publication No. 63-295695).
An organic EL element having such a conventional alloy cathode does not, however, have sufficient environmental stability and film formability; for example, in an organic EL element having the cathode made of an aluminum magnesium alloy, particularly, a non-light-emitting portion (hereinafter referred to as "black spot") is generated in the light-emitting area (1 dot: 2 .times.2 mm) and grows with time. The use of the conventional alloy cathode makes it difficult to increase the luminance with a low drive voltage, and cannot thus provide an organic EL element with high luminous efficiency and high luminance.